Washington (CNN) - House Speaker John Boehner's proposal to avert the looming fiscal cliff's automatic tax increases has failed to get enough Republican support, throwing yet another monkey wrench into the contentious debate.

Boehner said earlier Thursday that he was confident that his so-called Plan B - which would extend tax cuts that are set to expire at year's end for most people while allowing rates to increase to 1990s levels on income over $1 million - would pass the House, and in the process put pressure on President Barack Obama and the Democratic-controlled Senate. But that gambit seemed in doubt Thursday, as Republican leaders struggled to get all their members to sign on, knowing the chamber's Democrats oppose it.